The invention features devices and methods for enriching a sample in one or more desired particles. An exemplary use of these devices and methods is for the enrichment of cells, e.g., white blood cells in a blood sample. In general, the methods of the invention employ a device that contains at least one sieve through which particles of a given size, shape, or deformability can pass. Devices of the invention have at least two outlets, and the sieve is placed such that a continuous flow of fluid can pass through the device without passing through the sieve. The devices also include a force generator for directing selected particles through the sieve. Such force generators employ, for example, diffusion, electrophoresis, dielectrophoresis, centrifugal force, or pressure-driven flow.